1. Field of the Invention
The present invention relates to an endless belt device provided with an endless belt for bearing an image or a medium for forming an image or for pressing the medium, and an image forming apparatus using the endless belt device, such as a copier, a printer, a facsimile apparatus, etc.
2. Discussion of the Background
A tandem-type color image forming apparatus in which a plurality of photoconductors as image bearing members are arranged side-by-side obtains a full color image on a recording medium by transferring toner images formed on respective photoconductors onto an endless belt as an intermediary transfer member, sequentially, to be superimposed on top of each other and by transferring the superimposed toner images onto the recording medium in a lump.
In the image forming apparatus, the toner images formed on the plurality of photoconductors are transferred onto the intermediary transfer member, sequentially, starting with the one at the most upstream side in the direction in which the intermediary transfer member is conveyed, and the timings of transferring the toner images from respective photoconductors are sequentially delayed so that the transferred toner images are superimposed on top of each other. The speed of the intermediary transfer belt is detected with a detection device, and when a variation is caused in the speed, the timings of transferring respective toner images are appropriately adjusted.
Similarly, in an image forming apparatus in which toner images formed on a plurality of photoconductors are transferred onto a recording medium being born on and conveyed by an endless belt, sequentially, while being superimposed on top of each other, the speed of the endless belt is detected and the timings of transferring the toner images are adjusted.
Various methods of detecting the speed of an endless belt have been proposed. For example, Japanese Patent Laid-open publication 2002-251079 describes a method of providing a speed detection device (a light transmittance-type sensor) configured to generate a signal by transmitting and shielding a light, at a part of an end part of an axis of a driven roller driven by an endless belt, which is cut in a shape of “D”, and controlling the speed of the endless belt based on the information from the speed detection device.
Also, Japanese Patent Laid-open publication No. 4-234064 (Japanese Patent publication No. 3186090) describes a speed detection device using an encoder disk.
Further, Japanese Patent Laid-open publication No. 2001-306149 describes a method of detecting the speed of a rotary member such as a photoconductor drum by detecting a marker provided on the rotary member in the circumferential direction thereof, using a marker sensor constituted of a photo-interrupter.
In the speed detection method using an encoder disk, however, a detection mechanism protrudes from an end part of a rotation axis, and a disk with an axis having a diameter larger than that of the rotation axis is necessary for the detection mechanism, so that the detection mechanism is relatively large.
In the method of using a transmittance-type sensor at a D-shaped part of an end part of an axis, the detection mechanism is relatively large, similarly, and further, it is difficult to arbitrarily set the number of markers, so that adjustment of the detection accuracy is difficult.
In the method of providing a marker on a photoconductor drum in the circumferential direction thereof and detecting the marker by a photo-interrupter, the photo-interrupter cannot be close to the photoconductor drum due to a concern that the photoconductive property of the photoconductor drum is affected, so that the detection accuracy cannot be increased so much. Further, there is a concern that the charged marker electrostatically affects the surface of an endless belt arranged close to the photoconductor drum.
Further, generally, the detection region of a detection device is in an environment that toner and dust float, so that the concern exists that in the long term, the toner and the dust adhere to the detection device and the detection accuracy decreases.